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Lecture 3 GEOLOGICAL STRUCTURES AND MAPS
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Lecture 3 GEOLOGICAL STRUCTURES AND MAPS

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  1. Lecture 3GEOLOGICAL STRUCTURESAND MAPS A2.2GZ2 Geology for Life Sciences

  2. Overview • Geological structures - introduction • Formation of geological structures • Inclined planar beds • Unconformities • Folds • Faults • Igneous rocks • Geological maps • Interpreting published maps

  3. GEOLOGICAL STRUCTURES

  4. The term ‘geological structure’ is used to describe the three-dimensional relationship of layers of rock • The geological structure underground gives rise to the outcrop pattern of the beds at the surface • The geological map shows a representation of this outcrop pattern (perhaps simplified) • It is therefore possible to work out the underground structure from the map by applying some simple rules.

  5. FORMATION OF GEOLOGICAL STRUCTURES

  6. Geological structures form in response to stress • The type of structure so formed is controlled by • the magnitude and direction of the stress field • the mechanical properties of the rocks under the prevailing conditions of pressure and temperature • rate of stress (as stress rate is increased, the material goes from being elastic to plastic and finally fracturing by brittle fracture) • temperature (at lower temperatures, the material fractures sooner. At higher temperatures, the material behaves like a plastic longer before fracturing.) • confining pressure (the lower the pressure, the sooner the material will fracture) • Structures can be cms to kms in size.

  7. Compression Compression Faulting Folding • Rocks placed in compression will either fold or fracture, depending on their plasticity and/or their ability to slip relative to one another along their internal surfaces • Compression arises from large scale crustal movements due to plate tectonics.

  8. Rocks in tension will usually respond by fracturing • The tension often arises from lateral expansion due to consolidation or basin formation.

  9. INCLINED PLANAR BEDS

  10. Planar beds, either flat or inclined, are the simplest form of geological structure • They arise from simple deposition of horizontal layers, perhaps followed by tilting due to tectonic forces causing regional folding • On the larger scale most ‘tilted’ beds form a part of larger structures such as folds • By knowing the original orientation, we can infer what happened to layered rocks that are no longer horizontal.

  11. Planar beds

  12. We use three terms to describe tilted beds: • The true dip angle is the angle with the horizontal down the line of greatest slope • The dip direction is the bearing of the dip • The strike is the direction of outcrop, thus is perpendicular to the direction of dip. • If the rock is seen in a section oblique to the dip direction, the dip seen is the apparent dip. This is less than the true dip.

  13. B A • The outcrop pattern of a tilted bed depends on the angle of tilt and on the surface topography • If the ground surface is flat, the tilt angle is dominant. This controls the width of the outcrop. • Determining the thicknesses of tilted rock layers can be calculated, though, using trigonometry. Thickness = Surface Width * sin(dip angle) Thickness of beds A and B is the same but outcrop of B is much wider than A due to it’s shallower dip angle

  14. If the ground surface is not flat: • If the rocks are horizontal or just slightly tilted, the outcrop follows the contours of the land • "V"-shaped, or "horseshoe" patterns occur when a valley is incised into flat layers • If the rocks are strongly tilted, the outcrop becomes less dependent on the contours • In the extreme case of vertical beds, the outcrop cuts straight across the ground • The effect of the topography is less visible on smaller scale (larger area) maps

  15. If the rocks are horizontal or just slightly tilted, the outcrop follows the contours of the land

  16. Parallel stripes often indicate a terraced landscape with horizontal layers

  17. "V"-shaped, or "horseshoe" patterns occur when a valley is incised into flat layers

  18. If the rocks are horizontal or just slightly tilted, the outcrop follows the contours of the land

  19. horizontal rocks outcropping in a terraced mountain rocks tilted and eroded to a flat landscape It is important to realize that geologic structures(tilted or folded rocks)in areas offlat topographycan make similar patterns to those formed byhorizontal rocks in mountainous or canyonland (terraced) topography The Strike and dip symbols can be used to distinguish the two situations.

  20. In the extreme case of vertical beds, the outcrop cuts straight across the ground

  21. UNCONFORMITIES

  22. An unconformity arises when a series of tilted beds becomes buried beneath further beds of a different dip • They are surfaces representing a period of uplift and erosion between adjacent rock units • An unconformity thus divides a sequence into a lower series and an upper series of beds • The plane of unconformity is the surface across which the dip changes • Geologically it is the eroded surface of the highest bed in the lower series and is a gap in the sedimentary rock record.

  23. Plane of unconformity Unconformities are recognised on a geological map by the overstep of a younger outcrop across an older one

  24. Unconformity at Siccar Rock, East Lothian.

  25. Unconformities are recognised on a geological map by the overstep of a younger outcrop across an older one • This gives the appearance of younger beds covering up older ones

  26. FOLDS

  27. Folds occur as the response to a compressive stress • We define the fold axis (or hingeline), the axial plane, the limbs and the nose of the fold • We distinguish synclines and anticlines by the relative direction of the fold nose

  28. Nose The axis, or hinge, of the fold is the line of sharpest curvature of any given layer.

  29. The simplest fold is a upright structure with the beds remaining parallel. This structure is symmetrical about a vertical axial plane • This leads to a simple repetitive outcrop pattern in which the beds are mirrored about the axial plane or hinge line on an eroded surface

  30. Folds are a type of waveform and do not usually occur singly • We see folds as a sequence of synclines and anticlines • This is termed a fold train.

  31. More complex structures arise if the axial plane is itself tilted. This leads to a type of fold known as anasymmetric fold.

  32. In a more extreme case the axial plane is tilted such that both limbs dip in the same direction • This is termed an overfold or recumbent fold.

  33. In many cases the hinge line of the fold dips into the ground • This creates a plunging fold.

  34. The fold noses crop out successively along the line of plunge • Thus outcrop of the limbs of a plunging fold close across the hinge line • This leads to a distinctive curved outcrop and allows the hinge line to be plotted on a map

  35. If a fold closes in all directions we have either a dome or a basin • These can be recognised from their very distinctive closed outcrops.

  36. From the geometry it is easy to see that in a dome all the beds will dip outwards and the oldest rocks will be exposed at the centre (the surface has been eroded).

  37. In a basin the converse is the case: the the beds dip inwards and youngest rocks are found at the centre.

  38. Granton Barnton Blackhall Zoo Murrayfield • These closed outcrop patterns are easily visible on small-scale geological maps • In order to distinguish a dome from a basin we need to know either the relative ages or the dip directions.

  39. Over a larger area, domes and basins occur in fold trains and are often offset sideways from one another • They are in essence ‘wrinkles’ in a sheet of rock that has been pushed laterally

  40. FAULTS

  41. Faults are formed when the rock reacts to stress by brittle fracture • Three types can be recognised: • normal faults (dip-slip) • reverse faults (dip-slip) • wrench faults (strike-slip) • The type of fault is determined by the orientation of the principal stresses

  42. NORMAL FAULT Upthrow side Downthrow side In a normal fault, the block above the fault moves down relative to the block below the fault. This fault motion is caused by tensional forces and results in extension. [Other names: normal-slip fault, tensional fault or gravity fault].

  43. REVERSE FAULT In a reverse fault, the block above the fault moves up relative to the block below the fault. This fault motion is caused by compressional forces and results in shortening. A reverse fault is called a thrust fault if the dip of the fault plane is small. [Other names: thrust fault, reverse-slip fault or compressional fault].